Upon reproducing colors by a display, printer, and the like, color matching is normally made by a method of matching the tristimulus values of an original with those of an output on the basis of the trichromatic theory. A human being converts the spectral reflectance of an object as a continuous function in a visible wavelength range (about 380 to 780 nm) into responses (to be referred to as tristimulus values hereinafter) of three different cells called cones, which are distributed on the retina, and perceives colors of the object on the basis of the tristimulus values. As typical colorimetric systems used to quantify the tristimulus values, an XYZ colorimetric system and CIELAB colorimetric system are known. The XYZ colorimetric system is defined by:
                    X        =                  k          ⁢                                    ∫                              380                ⁢                                                                  ⁢                nm                                            780                ⁢                                                                  ⁢                nm                                      ⁢                                          S                ⁡                                  (                  λ                  )                                            ⁢                              R                ⁡                                  (                  λ                  )                                            ⁢                                                x                  _                                ⁡                                  (                  λ                  )                                            ⁢                              ⅆ                λ                                                                        (        2        )                                Y        =                  k          ⁢                                    ∫                              380                ⁢                                                                  ⁢                nm                                            780                ⁢                                                                  ⁢                nm                                      ⁢                                          S                ⁡                                  (                  λ                  )                                            ⁢                              R                ⁡                                  (                  λ                  )                                            ⁢                                                y                  _                                ⁡                                  (                  λ                  )                                            ⁢                              ⅆ                λ                                                                        (        3        )                                          Z          =                      k            ⁢                                          ∫                                  380                  ⁢                                                                          ⁢                  nm                                                  780                  ⁢                                                                          ⁢                  nm                                            ⁢                                                S                  ⁡                                      (                    λ                    )                                                  ⁢                                  R                  ⁡                                      (                    λ                    )                                                  ⁢                                                      z                    _                                    ⁡                                      (                    λ                    )                                                  ⁢                                  ⅆ                  λ                                ⁢                                                                  ⁢                for                                                    ⁢                                  ⁢                  k          =                      100                                          ∫                                  380                  ⁢                                                                          ⁢                  nm                                                  780                  ⁢                                                                          ⁢                  nm                                            ⁢                                                S                  ⁡                                      (                    λ                    )                                                  ⁢                                  y                  ⁡                                      (                    λ                    )                                                                                                          (        4        )                            S(λ): spectral distribution of illumination        R(λ): spectral reflectance of object         x(λ), y(λ), z(λ): color matching functionsThe CIELAB colorimetric system is defined by:        
                              L          *                =                              116            ⁢                          f              ⁡                              (                                  Y                                      Y                    n                                                  )                                              -          16                                    (        5        )                                          a          *                =                  500          ⁢                      {                                          f                ⁡                                  (                                      X                                          X                      n                                                        )                                            -                              f                ⁡                                  (                                      Y                                          Y                      n                                                        )                                                      }                                              (        6        )                                                      b            *                    =                      200            ⁢                          {                                                f                  ⁡                                      (                                          Y                                              Y                        n                                                              )                                                  -                                  f                  ⁡                                      (                                          Z                                              Z                        n                                                              )                                                              }                                      ⁢                                  ⁢                              f            ⁡                          (                              X                                  X                                      n                    ⁢                                                                                                                            )                                =                      {                                                                                                                              (                                                  X                                                      X                            n                                                                          )                                                                    1                        3                                                              ,                                                                  X                                                  X                          n                                                                    >                      0.008856                                                                                                                                                                                      7.787                        ⁢                                                  (                                                      X                                                          X                              n                                                                                )                                                                    +                                              16                        116                                                              ,                                                                  X                                                  X                          n                                                                    ≤                      0.008856                                                                                                                              (        7        )                            f (Y/Yn) and f (Z/Zn) are similarly calculated.Also, as a typical method of quantifying the difference between colors of two objects, color difference ΔE specified by the CIE (International Commission on Illumination) is known, and is given by:ΔE=√{square root over ((L1*−L2*)2+(a1*−a2*)2+(b1*−b2*)2 )}{square root over ((L1*−L2*)2+(a1*−a2*)2+(b1*−b2*)2 )}{square root over ((L1*−L2*)2+(a1*−a2*)2+(b1*−b2*)2 )}  (1)        
Upon color matching among an image input device such as a scanner, digital camera, or the like, an image display device such as a monitor or the like, and an image output device such as a printer or the like, color correction parameters and the like are optimized using equation (1) above so as to minimize color difference ΔE between the object and target colors.
On the other hand, when a human being perceives the colors of an object, the illumination condition largely influences such perception. In order to precisely reproduce colors under various illumination light sources, spectral reflectance characteristics must be matched (such process will be referred to as spectral color reproduction) in place of tristimulus values, and a color correction method that minimizes errors between spectral reflectance characteristics is known.
For example, Japanese Patent Laid-Open No. 09-163382 (U.S. Pat. No. 5,929,906) describes correction of color misregistration due to the characteristics of an image output device. According to this reference, color separation values are corrected using spectral reflectance in an intermediate colorimetric system. However, tristimulus values under a predetermined light source are used to optimize correction.
Also, Japanese Patent Laid-Open No. 05-296836 describes that evaluation for optimizing object colors is made using the square means (RMS error) of spectral distribution errors for respective wavelengths, which is given by:
                              (                      RMS            ⁢                                                  ⁢            Error                    )                =                              ∑                          λ              =                              380                ⁢                                                                  ⁢                nm                                                    780              ⁢                                                          ⁢              nm                                ⁢                                                                      {                                                            R                      ⁡                                              (                        λ                        )                                                              -                                          o                      ⁡                                              (                        λ                        )                                                                              }                                2                            n                                                          (        8        )            where R(λ) is the spectral distribution function of a color to be evaluated (to be referred to as an evaluation color hereinafter), and o(λ) is that of a target color,in place of the tristimulus value difference, and a color conversion process is executed based on this evaluation.
Furthermore, Japanese Patent Laid-Open No. 2001-008047 (EP1054560A) describes a method of executing a color conversion process by evaluating errors for respective wavelengths by a method of calculating the square mean after errors for respective wavelengths are multiplied by a weighting function generated from a CIE color matching function (to be simply referred to as a color matching function hereinafter) as visual characteristics depending on wavelengths.
However, upon conversion into, e.g., tristimulus values L*a*b*, since conversion into three stimulus values is made using the spectral reflectance of an object as a continuous function in a visible wavelength range (about 380 to 780 nm), different spectral distributions are often converted into identical tristimulus values. For this reason, even when tristimulus values match those of an original under a given illumination, a change in illumination light source brings about a different change in tristimulus values, and original and reproduction colors have different color appearances.
For example, two spectral reflectance characteristics shown in FIGS. 13A and 13B are converted into equal tristimulus values under CIE supplementary standard light D50, but into different tristimulus values under CIE standard light A. That is, even when the color difference between two objects becomes zero under a given light source, metamerism is effected under only that condition, and the color difference may increase under another light source.
In Japanese Patent Laid-Open No. 09-163382 that discloses the technique associated with correction of color misregistration due to the characteristics of an image output device, color separation values are corrected using spectral reflectance in an intermediate colorimetric system, but tristimulus values under a predetermined light source are used to optimize correction. For this reason, a change in light source results in a change in optimization result.
In the method of making evaluation using the square mean (RMS error) of spectral distribution errors for respective wavelengths, as described in Japanese Patent Laid-Open No. 05-296836, no problem of matching of colors due to metamerism occurs, but a simple square mean of errors for respective wavelengths of the spectral distribution is used, and light source information and visual characteristics are not taken into consideration. Therefore, the color difference may increase even when two colors have close spectral distributions. For example, if the spectral distribution of an original is as shown in FIG. 14A, a spectral distribution in FIG. 14B has a smaller RMS error than that in FIG. 14C. However, under CIE supplementary standard light D50, the spectral distribution in FIG. 14C has smaller ΔE, and color appearance of FIG. 14C is closer to the original color (FIG. 14A) than FIG. 14B. Hence, the evaluation results and color appearance have gaps.
Furthermore, Japanese Patent Laid-Open No. 2001-008047 considers neither light source information nor visual characteristics having nonlinearity with respect to brightness. For this reason, the same weight is used independently of the contrast (spectral distribution shape) of an object. As a result, a color with the best evaluation value does not always have a minimum error of color appearance.